1. Field of the Invention
The present invention relates to frangible, ceramic conveyor rolls designed for use in the heat treatment of glass sheets wherein the rolls are rotated to transport a series of glass sheets through a hot enclosed atmosphere.
Furnaces for tempering or annealing glass conventionally include conveyors which carry glass sheets through an enclosed atmosphere of high temperature during the annealing or tempering of the glass. A so-called roller hearth-type of conveyor utilized to carry glass sheets includes horizontally extending conveyor rolls of ceramic material on which heated glass sheets are rotatably supported. Rotating the conveyor rolls drives the glass sheets by friction over the rolls through an enclosed furnace atmosphere of high temperature.
One ceramic material utilized to form cylindrically shaped rolls for a roller hearth is a sintered fused silica material that is made by grinding solid fused silica into small particles and then sintering these particles to each other at a temperature below the fusing temperature. Such a material is readily formed into an elongated shape required for the rolls of the roller hearth. In addition, fused silica rolls so formed have a relatively small coefficient of thermal expansion. This latter characteristic is extremely desirable due to extreme temperatures to which the rolls in the roller hearth are subjected during use.
One conventional way for rotatably supporting and driving heated glass conveyor rolls of fused silica and other ceramic compositions utilizes bearings that support the opposite end portions of the rolls in the roller hearth. A drive member having a toothed drive component such as a gear or a chain sprocket drives the roll at one end. A metallic end cap rotatable with a driven shaft is conventionally used to transmit the drive from the drive gear or chain sprocket to the end of the ceramic roll.
Rolls of ceramic composition are superior to metal rolls in high temperature environments for several reasons. First of all, ceramic rolls have a low coefficient of thermal expansion compared to that of metal rolls so that they are less likely to warp. Ceramic rolls are less likely to oxidize and flake off and develop pock marking in their glass engaging surface. However, ceramic materials are difficult to work and are quite frangible. Furthermore, such rolls composed of ceramic material cannot be connected directly to the drive mechanisms needed to rotate the rolls. The use of metal end caps with ceramic rolls for roller hearths has been developed to a certain extent. However, they have introduced problems of securing the metallic end caps to the ends of the ceramic rolls that they drive in transmitting a torque from the driving mechanism. One method involves an adhesive bond between the end cap and the end of the ceramic roll. Another method involves shrinking a metallic end cap onto the roll end by heat shrinking. Both of these methods have drawbacks. Adhesives presently known require an extended cure period of at least about 24 hours before the ceramic roll is securely fixed to its end cap. Such a delay is too long to accept for mass production operations.
Heat shrinking requires heating the metallic end cap to a very high temperature so that its diameter increases to enable it to be shrunk fit over the end of the ceramic roll. The internal diameter of the end cap and the outer diameter of the ceramic roll must be closely matched so that the end of the roll is small enough to be inserted into the heated end cap but large enough to be securely clamped after cooling. Often it is necessary to machine one or both of the matching parts in order to provide suitable matching. At the inner end of the metallic cap, the ceramic roll is subjected to a clamping pressure due to the heat shrinking of the end cap. Adjacent to this portion of the ceramic roll subjected to the clamping pressure is an adjacent portion free of the clamping pressure. At this location of the boundary between a high clamping pressure and no clamping pressure, ceramic rolls are subject to fracture due to the high stresses involved.
Fused silica and other ceramic composition rolls are more fragile than metallic rolls. Hence, devices suitable for rapidly coupling and uncoupling metal driving shafts to metal conveyor rolls for the purpose of transmitting rotating forces from metal driving shafts to rotate metal rolls used to convey sheet materials by rolling friction have been considered impractical for use with ceramic rolls. Connectors between rotating metal elements such as between a wheel hub and an axle or spindle thereof or between metal elements of pulleys, gears, sprockets or sheaves and comprising split tapered sleeves adapted to be tightened and contracted on a metal shaft by an axially movable element which is clamped by cap screws or bolts and readily released by the use of jack screws have been limited in the past to metal structures in the belief that ceramic materials such as fused silica do not lend themselves to clamping action by metal gripping members because of the inherent frangibility of ceramic materials. Hence, there existed a need to provide means for attaching ceramic conveyor rolls to metal drive shafts that did not cause a metal to ceramic connection of the type that would cause the metal to induce breakage of the fragile ceramic rolls and to provide ready disconnection between the ceramic roll and the metal drive shaft whenever the occasion required same.
2. Description of the Prior Art
U.S. Pat. No. 2,152,076 to Menough discloses metal alloy conveyor rolls having end portions provided with axially extending slots at their inner portions and a center portion provided with axially extending slots at their end portions. Welds are provided in the axially extending slots of the center portion to connect the end portions thereto. The end portions are inserted within the ends of the center portion. The axially extending slots of the end portions permit the end portions to expand as the center portion expands radially due to an increase in temperature, even though the end portions do not heat as rapidly as the center portion of the rolls.
U.S. Pat. No. 3,608,876 to Leaich et al discloses pin and slot connections between ceramic hearth rollers and metal shafts. The fragility of ceramic rolls makes such pin and slot connections impractical.
U.S. Pat. No. 3,867,748 to Miller discloses the use of an adhesive for securing metal end caps to the ends of cylindrical ceramic conveying rollers in a roller hearth. The rolls cannot be used until the adhesive is cured. Since curing is a slow process, such rolls are not practical for rapid installation for high-speed production operations.
U.S. Pat. No. 4,131,420 to Miller utilizes a two-piece coupling that disconnects a ceramic roll including its end cap from a drive train which provides a rotating force to the end cap of the ceramic roll. The end cap in this patented apparatus is attached to the ceramic roll using an adhesive in a manner similar to that of the previous patent.
U.S. Pat. No. 4,140,486 to Nitschke utilizes a metallic spring end cap for rotatably driving a glass conveyor roll of ceramic material using helical spring coils that have a diameter in an undeflected condition slightly less than the diameter of the end of the ceramic roll so that coil deflection increasing the coil diameter allows mounting the coils over the end of the ceramic roll and a subsequent release clamps the coils over the end of the roll. The helical spring coil may be engaged by friction or by tooth sprocket or gear driven rolls to transmit a force thereto.
Other patents reported in a novelty search include U.S. Pat. No. 766,261 to Jebsen, which discloses a drive shaft provided with spring fingers; U.S. Pat. No. Re. 19,205 to Blythe, which discloses a flexible coupling between a conveyor roll and a drive shaft that consists of a collar or sleeve telescoped over the adjacent ends of the associated drive shaft and roll shaft and loosely connected therewith by bolts arranged transversely of each other; U.S. Pat. No. 1,395,913 to Ford that shows a metal hub construction in which a split tapered bushing has ribs that are received in a plurality of longitudinal and circumferentially disposed grooves or ways in the outer end of an axle and tightened about the axle when screw bolts draw the hub and bushing together; U.S. Pat. No. 1,872,062 to Burke, which shows means for connecting end roll sections to a center roll section in a manner that allows for different expansion and contraction thereof; U.S. Pat. No. 2,407,032 to Myers, U.S. Pat. No. 2,482,662 to Dunne and U.S. Pat. No. 2,612,395 to Russell all show longitudinally split sleeves adapted to be interposed between the hub and shaft of a pulley and provided with a tapered periphery to adjust the radial clamping force between the shaft and the hub on axial movement of the hub; U.S. Pat. No. 2,634,144 to Friedman shows a metal roll secured to a roll shaft by a tapered split sleeve arrangement which permits adjustment of the roll axially with respect to the roll shaft; U.S. Pat. No. 2,788,957 to Lindquist, which shows a composite refractory roller comprising hollow refractory sections under compression; U.S. Pat. No. 3,489,397 to Alexander, which discloses a frictional drive between a ceramic rod and a sleeve that comprises a substantially U-shaped spring that is mounted on a pin that is attached to the hub of a sprocket and that has spring legs received in slots formed on opposite sides of the sleeve so that the legs frictionally engage the rod; and U.S. Pat. No. 4,034,837 to Vinarcsik et al that discloses flanged metal rolls secured to mating flanges on stub shafts by bolts and separable from one another by jack shafts.